13
of Donahue and co-workers
18
that the success rate with drainage implants for aphakic
glaucoma control was greatest in those eyes that had had only one previous operation.
Our success rate with the Ahmed Glaucoma Valve also compares favorably with
recently reported success rates of other surgical methods in the treatment of pediatric
glaucoma such as trabeculectomy with adjunctive mitomycin C (59-67% at 12-24 months),
23-
24
cyclocryoablation (66% and 44% success after 6 and 57.6 months respectively)
25
and trans-
scleral YAG laser (50% at 15 months after retreatments in 70% of their patients).
7
The success rate in our patients fell to 71% and 46% after 36 and 48 months
respectively. We can not attribute this to mal- distribution of follow-up rates as there was no
statistically significant difference in follow up between success and failure groups Likewise,
although the Canadian health care system may cause more patients who do well to be
discharged to community physicians away from our tertiary care center, we made every
attempt to obtain ongoing follow-up information and have little evidence that this contributed
to the fall-off in our reported success rates. Hamush et al had similar rate of success over
time.
12
Therefore, we can only assume that the fall-off in success rates over times is a real
observation reflecting what one might expect as a long-term outcome for this implant.
Post-operative hypotony can be a major problem with all aqueous drainage implants
affecting both the short and long-term surgical outcome. Laboratory studies have shown that
the Ahmed Glaucoma Valve, as opposed to the Optimed (Kowa Optimed, Torrance, CA,
USA) or Krupin (Hood Laboratories, Pembroke, MA, USA) implants, has the best flow
restricting function and closely regulates pressure within a desired range.
22
In one of our
patients, we fortuitously had the opportunity to observe that IOP did not drop below 5 mmHg
despite the fact that the implant was in direct external communication as the result of a
wound dehiscence over the plate following failed excision of a Tenon cyst. Only the Ahmed
implant and the upper eyelid could have acted as barriers to aqueous egress.
